gprs: Move log message about mm context after NULL check

Fixes: CID#1240205
diff --git a/openbsc/src/gprs/gprs_gmm.c b/openbsc/src/gprs/gprs_gmm.c
index a4305e8..ec639dc 100644
--- a/openbsc/src/gprs/gprs_gmm.c
+++ b/openbsc/src/gprs/gprs_gmm.c
@@ -582,14 +582,14 @@
 	char mi_string[GSM48_MI_SIZE];
 
 	gsm48_mi_to_string(mi_string, sizeof(mi_string), &gh->data[1], gh->data[0]);
-	LOGMMCTXP(LOGL_DEBUG, ctx, "-> GMM IDENTITY RESPONSE: mi_type=0x%02x MI(%s)\n",
-		mi_type, mi_string);
-
 	if (!ctx) {
 		DEBUGP(DMM, "from unknown TLLI 0x%08x?!? This should not happen\n", msgb_tlli(msg));
 		return -EINVAL;
 	}
 
+	LOGMMCTXP(LOGL_DEBUG, ctx, "-> GMM IDENTITY RESPONSE: mi_type=0x%02x MI(%s)\n",
+		mi_type, mi_string);
+
 	if (mi_type == ctx->t3370_id_type)
 		mmctx_timer_stop(ctx, 3370);